This invention relates to methods and apparatus for converting pneumatic to electrical signals and conversely, and more particularly, relates to a general purpose pneumatic to digital converter system adaptable to process control and pressure monitoring systems for boilers, paper mill plants, refineries, or the like.
In such systems, it is frequently desirable to control or monitor pressures at a plurality of various critical points such as valve ports located along the process. Moreover, it is conventional to convey these pressures to a central control chassis by means of charged pneumatic lines. The chassis is provided with appropriate pressure transducers and circuitry for processing and conditioning the transducer analog voltage outputs for delivery on electrical lines from the chassis to a desired location, as for example, to a host process control computer.
One problem with such approaches is that transducers are frequently in need of replacement or recalibration due to burnouts, changes in characteristics such as offset, gain or transfer function non-linearities due to temperature effects, aging, etc., and the like. In such instances, the particular pressure line involved is shut off and disconnected from the chassis, the associated transducer is removed and repaired or replaced, the pressure line is thence reattached, and the pressure point is thus able to be monitored again.
However, several severe problems are associated with this approach. First, it is not unusual to seek to measure or control as many as 32 or more pressure points in a typical application, and it will be readily apparent that it can become quite time consuming to perform for each such point the aforementioned steps of shutting off the pressure line associated with the point, disconnecting the line and associated transducer, replacing the transducer, reconnecting and then recharging the pressure line. Moreover, the possibility is manifest for introduction of leaks due to faulty connection of numerous pressure lines, or worse yet, errors in reconnection of pressure lines to incorrect locations on the chassis due in part to the large number of connections.
Still more serious, however, is the fact that during such disconnections, it is conventional to arrange for manual monitoring and control of the associated pressure points during transducer changes. Such monitoring often is unreliable, particularly in view of the fact that there may be inordinately long delays between the cessation of the automatic monitoring and transition to manual monitoring during transducer replacement. This, in turn, often leads to certain critical process points being left uncontrolled or monitored poorly (if at all). The result is production of bad product, for example, when critical set points in the process are permitted to stray, or more seriously, the creation of dangerous situations as, e.g., in the case of control of high pressure boilers, high temperature processes, or the like.
Unsuccessful attempt has been made to solve some of the aforementioned problems as, e.g., by various measures employing a combination of "quick connect" type electrical edge tab connectors and pneumatic self-seal connectors. For example, in the patent to More, U.S. Pat. No. 4,133,022, a rack assemblyisprovided as having a plurality of modules, each module carrying a pneumatic connector and electrical edge tab connector, both such connectors being adapted to quickly interconnect with mating connectors in the rack assembly when the module is plugged into it. However, such efforts have failed to provide a satisfactory general purpose pneumatic to digital converter system for various reasons.
First, there is frequently no provision for speedy and accurate registry of alignment of the pneumatic and electrical connectors to provide for reliable simultaneous interconnection of both connectors during module replacement. In other systems, there is further no provision for convenient electrical intercommunication between modules. This may be particularly important when it is realized that in the chassis it may be desirable to provide additional circuit board modules which interrelate and may interconnect by means of standard compatible bus formats to those carrying pressure transducers--such additional modules being provided for signal preprocessing for pressure limit checks or the like, for communication, memory, disc drive, or any of a number of desired functions.
Even if such interactive modules carrying pressure transducers could be provided which may be easily removed and replaced reliably and quickly, yet another problem relates to the characteristics of transducers themselves and associated analog to digital converters or other circuitry which may be desirably utilized therewith. More particularly, such transducers, converters, and other circuitry desirably located in a module may introduce inaccuracies which must be compensated for due to their particular offsets, gains, or transfer function characteristics which may exhibit non-linearities due to temperature changes or the like. While conventional approaches may have provided, in the form of look-up tables, some form of memory located in a given module for providing such compensation data automatically with the transducer as the module is substituted or plugged in, typically, the data must first be derived from the module circuitry which includes the associated transducer.
Then, in an additional separate step, this data is stored or burned in to a suitable memory device located remotely of the module, the memory device then later being installed in the module itself in yet a further step. The requirement of the separate steps of physically deriving data from within the module, storing it in a memory device remote from the module, and then installing the memory device in the module not only has been found to be time consuming, particularly in the tpyical case wherein multiple transducer modules are employed, but has been found to be unreliable as well.
These disadvantages of the prior art are overcome with the present invention, and a general purpose pneumatic-digital converter is herein provided for effecting, by means of a plug-in module transducer construction, quick and reliable change-outs of pressure transducers having improved abilitites for such modules to be calibrated and compensated. Further, such a converter is provided which affords flexibility in electrical interaction between such modules and any other desired circuitry resident on auxiliary printed circuit boards by means of a printed circuit motherboard into which the modules and auxiliary boards are plugged having standard bus compatibility.